Magnetic testing apparatus



Aug. l2, 1947. R. w. BROWN 2,425,361

MAGNETIC T`r'1S'.|.]N(.`1` APPARATUS v Filed nec. e'. .1942

y 26 f- 2a HrTae/VEY` Patented Aug. 12, 1947 UNITEDl MAGNETIC TESTING APPARATUS Raymond W. Y Brown, Chicago, Ill., assigner to Western Electric Company, Incorporated, New York, N. Y., a corporation of New York c, H

Application December 8, 1942, Serial No. 468,241

2 Claims. l

This invention relates to electrical testing apparatus, and more particularly to anV apparatus for determining magnetic flux.

In the manufacture of certain types of communication equipment, and particularly in the manufacture of telephone receivers, it is sometimes desirable that the permanent magnets used therein be held to close tolerances as to magnetic flux. Since the flux of these magnets 'is sometimes relatively low, some diiiiculties may be experienced in accurately determining th flux.

I t is an object of the present invention to provide an effective and efiicient apparatus for determining magnetic flux.

'In accordance with one embodiment of this invention, an apparatus for testing the magnetic flux of magnets may be provided having a pair of rotating search c`oils connected to a milliammeter throughv an amplifying circuit. A testing circuit is associated with the amplifying circuit l for checking its output.

Other objectsl and advantages of` the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is`a schematic view of a circuit conl structed in accordance with this invention;

Fig. 2 is a plan view of the search coil; Fig. 3 is a sectional view along the line 3-3 of Fig. 2, and

Fig. 4 is a diagrammatic perspective view of the coils andthe associated conducting rings, shown in spaced relation.

This apparatus is intended to measure the flux of Small, relatively weak magnets and to provide a self-contained circuit for Calibrating the ap` Referring now to the drawings, and

. set screw I3.

The search coil comprises a pair of coils I4 and I5 wound about a bifurcated upper portion I6 of a laminated core structure I1', the lower portion I 8 of the core I1 being reduced and extending into a cylindrical block I9 of an insulating material such as phenol fiber or hard rubber. The core I1 is held in position in the block by bolts 20 and 2| which extends laterally through the block as shown.

vA sleeve 24 of brass or similar conducting material isv mounted around the upper portion of the block I3 and is connected; as shown in Fig. 4, to the left hand coil I4. A connector ring 25 of 2 material similar to the. sleeve 24 is mounted on the lower portion of the block I9 and is spaced from the sleeve 24. The right hand coil I5 is connected to this ring. It will be noted that the upper portion of the sleeve 24 is provided with a slot 26. By slotting the upper portion of the sleeve 24, the effect of a short circuited turn coupled to coils I4 and I5 is prevented and distortion due to eddy currents is substantially eliminated.

In the operation of this apparatus, a magnet 21 to be tested is placed on a holder 28 of ber or other non-magnetic material and is positioned a short distance above the upper surface of the laminated core structure I1, as shown in Fig. 3. The motor I2 is then started by closing a line switch 30. Closing this switch also supplies power to an amplifier 3| of standard design provided with the usualvampliiication control knob 29. A

pair of lbrushes orspring contacts 32 and 33 con-- nected to the amplifier 3l engage the sleeve 24 and ring 25, respectively, and the current generated by rotating the coils I4 and I5 in the field of the magnet 21 being tested is passed through the amplifier 3| and then through a transformer 36 and a normally closed switch 38 to a rectifier 34 whereby the current is converted to direct current. A milliammeter 35 is connected to the rectifier and indicates the amount of amplified current flowing therethrough.v By comparing the indications of the milliammeter with a standard, the flux of any particular magnet may be closely evaluated.

In the course of use, the tubes in the amplifier gradually become somewhat less efficient and other changes may also occur in the circuit which will affect the output of the amplifier. In this apparatus, however, a calibrating circuit is provided for accurately checking the amplifier oute :meter and then a current of .01` milliampere applied, 1 to 1000 being the desired amplification ratio. The amplifier is then adjusted to give the same milliammeter reading for the second as for the first current. '1"

Referring now to Fig. 1, in calibrating the output of the amplifier 3l the switch 38 is operated to disconnect the amplifier from the milliam- 1 meter and to complete a circuit through a variable resistor 39 and step-down transformer 40 from the line supply tothe milliammeter. The variable resistor' :il 1s adjusted to give the desired. deflection on the milliammeter and the current noted. The second switch Il is then operated to connect a second variable resistor l2, which is connected inparallel across the line as shown in Fig. 1, to the amplier and the first switch 38 is returned toits original position. It will be understood that no magnet is placed inv testing position during the calibration of the ampliiier. The setting ofthe variable resistor 42 is previously adjusted to reduce the current applied to the amplicore and coils serving as a shield for said coils -mounted on said block and connected to one of search -coil comprising a( laminated cylindrical fier to a predetermined fraction of the current y applied originally through the resistor I9 directly to the milliammeter. The amplier is then adfjusted to cause the second current to give the same reading on the milliammeter as thef first; the ratio of amplification will, thus. be equivalent to the quotient of the two currents. In practice,

once the ratio between the two currents has been established, it is not changed and, thus, since the resistors aresubstantially stable, the output vof thefampliiler may be easily and accurately checked. The switch. Il is then returned to its original setting and the apparatus is ready for testing.

While this apparatus is intended particularly for testing magnets having relatively low ilux, it will be understood that the apparatusis suitable for testing high flux magnets as well and will give extremely accurate results. v

While but one embodiment of this invention has been shown and described, it will be understood that many changes and modifications may be made therein without departing from the spirit or scope of the present invention.

What 'is claimed is:

1. In an apparatus for testing magnets, a search coil comprising a laminated cylindrical core having a bifurcated upper portion, a coil of semi-cylindrical shape wound about each part of the core, said coils being interconnected, a block of insulating material for supporting said core,

a sleeve of conducting material surrounding said core having a bifurcatedupper portion, a coil of semi-cylindrical shape wound about each part of h the core, said coils being interconnected, a blocky of insulating material for supporting said core, a sleeve of conducting material surrounding said core and coils serving as a shield for said coils mounted on said block and connected to one of said coils, the upper portion of said sleeve being slotted toeliminate distortion due to eddy cur-f rents, and a ring oi' conducting material mounted on said block and separated from said sleeve, sads ring being connected to the other of said co Y RAYMOND W. BROWN.

REFERENCES crrap UNITED STATES PATENTS Number Name Date 1,801,328 Burrows Apr. 21,- 1931 1,270,100 Ballman 'June 18, 1918 1,851,818 Drake et al. Mar. 29, 1932 2,036,856 Drake Apr. 7, 1936 1,317,786 -Hartshorn ,Oct. 7, 1919 1,094,106 White Apr. 21, 1914 974,760 Eastwood Nov. l, 1910 1,731,239 Aii'el Oct. 15, 1929 2,026,060 Pratt Dec. 31, 1935 2,170,515 Dionne et al Aug. 22, 1939 OTHER REFERENCES. Magnets, rny c. R. Underhill; McGraw-H111,

1924,l pp. 48-49. 

